1. Field of the Invention:
The present invention relates to a process for selectively separating two or more fluid (liquid or gaseous) molecular species from a mixture thereof utilizing solid particulate monomeric cyclodextrin which functions as a reversible sorbent for at least one of the molecular species.
2. Description of Related Patents:
Many techniques are known for separating liquid and gaseous molecular components from mixtures thereof. Typical such separation techniques take into account the differences in one or more physical properties of the respective components such as boiling point, polarity, molecular size, and/or molecular shape. For example, distillation is known to effect separations primarily based on boiling point differences of the components. In gas chromatographic separation processes which employ, as a stationary phase, a sorbing material such as a porous organic polymer or an inert solid packing coated with an organic liquid, the components are separated by their respective affinities for the polymeric sorbent or liquid coating as well as by boiling point.
A particularly common method for separating components having similar structures involves use as adsorbents of a certain class of uncoated solid separating materials, namely, porous crystalline aluminosilicates otherwise known as zeolites, as described, for example, by M. N. Y. Lee, Recent Developments in Separation Science (N. Li, et al., ed.) Vol. I, "Novel Separation With Molecular Sieves Adsorption", CRC Press, Inc., Boca Raton, 1972, pp. 75-112.
While zeolites are useful in effecting selective separations of certain hydrocarbons and isomers, they are limited in several respects, notably in their ability to achieve separations of particular importance to the petroleum and chemical industries. Thus, for example, whereas several zeolite materials are effective for separating normal alkanes of small molecular diameter (e.g., about 4.5-5 .ANG.) from other hydrocarbons, compounds of relatively large molecular size often cannot be separated by such zeolite materials. In addition, many zeolites are incapable of readily separating branched alkanes of similar structure from each other. Those separations which can be successfully carried out by using commercially available zeolites are strongly inhibited in the presence of water, so that care must be taken to avoid exposure of any part of the separation system to moisture during the separation process.
Cyclodextrins, which are cyclic oligosaccharides composed of alpha-1,4-linked glucose units arranged in a tours, are known to form inclusion complexes with a variety of "guest" molecules, i.e., molecules which are taken up by the "host" molecule, cyclodextrin. In addition, cyclodextrin and its derivatives are used to catalyze various homogeneous reactions and have been widely studied as models for enzyme catalysts.
Many of the cyclodextrin inclusion complexations observed in the past were conducted in aqueous or nonaqueous cyclodextrin solution. Thus, for example, Japanese Kokai Nos. 96530, 151804, 151827, 151833 and 2042825 disclose selective precipitation and separation of hydrocarbons with cyclodextrin solutions. Chromatographic separations by selective complexation of dissolved guests with insoluble, crosslinked cyclodextrin polymers in the presence of liquid water are disclosed in U.S. Pat. No. 3,472,835, U.K. Pat. No. 2,030,574 and by Y. Mizobuchi, et al., J. Chromatogr., 208, 35 (1981) and J. Szejtli, Staerke, 30, 127 (1978). More recently, B. Siegel and R. Breslow, J. Am. Chem. Soc., 97, 6869 (1975) reported that cyclodextrins form inclusion complexes in certain polar solvents such as dimethylsulfoxide.
It is known that guests can desorb from crystalline cyclodextrin complexes on washing with solvent, on heating or on allowing to stand, as reported by H. Schlenk and D. Sand., J. Am. Chem. Soc., 83, 2312 (1961), G. Phillips and P. Baugh, J. Chem. Soc., 387 (1966), J. Szejtli, et al., Acta Chim. Acad. Sci. Hung., 101, 27 (1979); J. Szejtli and Z. Budai, Acta Chim. Acad. Sci. Hung., 94, 383 (1977); and M. Maciejewski, et al., J. Macromol. Sci. Chem., A13, 87 (1979). It is further known that crystalline cyclodextrins will sorb guests. D. French reports in his Ph.D. Thesis, Iowa State University, 1942, that only one of five crystal modifications of alpha-cyclodextrin complex, i.e., the one with a channel lattice structure, sorbed iodine vapor. H. Schlenk, et al., J. Am. Chem. Soc., 77, 3587 (1955) discovered that alpha- and beta-cyclodextrins sorbed trichloroethylene and bromobenzene vapors, but only in the presence of water vapor or water of crystallization. A CPC International product information brochure entitled "Beta-Cyclodextrin", Englewood Cliffs, N.J., 1968 confirms Schlenk's work and concludes that water is necessary for appreciable cyclodextrin complexation. T. Kuge and K. Takeo, Agr. Biol. Chem., 32, 753 (1968) report that several crystalline cyclodextrin inclusion complexes fail to sorb such compounds as toluene and butanol, but when beta-cyclodextrin. propanol complex is heated to vacuum, peak broadening on a gas chromatography column is observed. In addition, alterations in polymeric membrane selectivity and flux were observed by C. Lee., Sepn. Sci. Technol., 16, 25 (1981) and J. Appl. Polym. Sci., 26, 489 (1981) on addition of cyclodextrins to the aqueous membrane casting solution. This was explained as possibly due to selective diffusion of organic permeant through cyclodextrin channels.
U.S. Pat. No. 3,472,835 discloses use of solid crosslinked cyclodextrin polymers, which selectively sorb gas phase components of cigarette smoke or flavor components, to effect petroleum separations. Liquid acylcyclodextrins are reported by D. Sand, et al., Anal. Chem., 33, 1624 (1961) and by H. Schlenk, et al., Anal. Chem., 34, 1529 (1962) to separate fatty acid derivatives.
Recently, Y. Mizobuchi, et al., J. Chromatogr., 194, 153 (1980) and Y. Mizobuchi, et al., Bull Chem. Soc. Jpn., 54, 2487 (1981) disclose that cyclodextrin polyurethane resins, obtained by polymerization of cyclodextrins with diisocyanates, selectively sorb various organic compounds in the absence of solvent. The 1980 article further discloses that native beta-cyclodextrin reversibly sorbs benzene under similar conditions.
It has now been discovered that shape-selective separations of molecular species, especially organic compounds, from a mixture thereof can be achieved in a continuous or batch process by using monomeric cyclodextrin in the solid particulate form as a reversible sorbent for at least one of the components of the mixture.
The solid particulate cyclodextrins used herein are particularly suited for achieving separations for which zeolites are ineffective. For example, unlike zeolites, the monomeric solid particulate cyclodextrin is found to sorb guest molecules of much larger diameter than the crystallographically measured free diameter of the host molecule cavity. In addition, effective separation of olefins and of branched alkanes can be carried out by the process herein.